The present invention relates to thermosetting powder coating compositions and, more particularly, to powder coating compositions employing a co-curing system.
Powder coating compositions for use in painting are extremely desirable. Such coating compositions greatly reduce and can even eliminate organic solvents used in liquid paints. When powder coating compositions are cured by heating, little, if any, volatile material is given off to the surrounding environment. This is the significant advantage over liquid paints in which organic solvent is volatilized into the surrounding atmosphere when the paint is cured by heating.
Powder coating compositions can be formulated with a carboxylic acid-functional acrylic polymer or a carboxylic acid-functional polyester. With such acid-functional polymers, the curing mechanism can be based upon one or more epoxy functional materials, e.g., a diglycidyl ether of a bisphenol or a glycidyl ether of a novolac resin. Although powder coating compositions using these materials give coatings with good properties, those including a novolac epoxy are expensive to formulate. Further, novolac epoxies tend to have low glass transition temperatures and, as a result, those powder coating compositions often have poor powder stability. Finally, novolac epoxies tend to result in yellowing of the powder coating compositions due to the action of heat and/or light.